Directional antenna



M y 1951 J. H. GARDNER 2,555,123

DIRECTIONAL ANTENNA Filed Ma rch 22, 1945 FlG.4

lNVENI'OR JOHN H. GARDNER Patented May 29, 1 951 2,555,123 DIRECTIONAL ANTENNA John H. Gardner, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application March 22, 1945, Serial No. 584,229

11 Claims. (01. 25033.65)

This invention relates to antennas for radio object-locating systems and particularly to means for obtaining a radiation pattern closely approximating the relationship csc o in one plane where is the angle measured from the axis of the radiating means.

In certain airborne radio object-locating systems it is desirable that the antenna afford a distribution of radiant energy in a relatively wide angle such that the variation of energy density versus the radiation angle is a cosecant-squared function, thereby providing relatively constant echo strength for targets located close in or distant. Antennas previously designed for afford ing such an energy distribution tend to produce upper lobes which cause interference patterns to appear on an indicator scope due to reflections from the belly of the airplane, and have relatively low gain. However, in airborne sets designed for use at medium or low altitudes such as for navigation the primary interest is long range with highgain.

Accordingly, it is one of the objects of the present invention to provide an antenna which is adapted to afford an energy distribution combining high gain with close approximation to a 050 0 beam pattern.

Another object of the invention is to afford a cosecant-sq-uared radiation pattern by the use of a modified paraboloidal antenna.

Another object is to achieve simplicity in the design of an antenna which is reliable and easy to operate.

Still other novel features and advantages will be apparent from the following description of the invention.

In the drawings:

Fig. l is an oblique view of an antenna, illustratin one embodiment of the present invention;

Fig. 2 is a partly diagrammatic vertical sectional view of the antenna of Fig. 1;

Fig. 3 is a partly diagrammatic view showing the radiation pattern produced by the antenna of the invention.

Fig. 4 is an outline drawing of the reflecting surface of the antenna showing a preferred form thereof. I

Referring to Figs. 1 and 2, the radiating element or feed l0, such as a dipole or preferably a double dipole with a pressurizing cup, may be constructed and located in a conventional manner'for illuminating a reflector ll. Reflector II in the present instance is formed in two parts. The upper portion l2 extending above and below thelevel of the radiator H] as viewed in Figs. 1

2. and 2 is a portion of a paraboloidal surface, which may be termed a dish, having its focal point substantially at the apparent center of radiation of radiating source ID. The lower portion l3 of reflector H is a portion of a cylinder disposed with its axis intersecting the axis of the upper portion [2. The spade-shaped cylindrical lower portion l3 has a curvature which is preferably that of a parabola but may, if desired, be that of a hyperbola or ellipse, disposed at a position relative to that of upper portion I 2 determined by the radiation pattern desired, as more fully described hereinafter.

The term parabolic cylindrical curvature as employed in this specification has the usual meaning of the curvature of a surface traced by a straight line moving parallel to a fixed line along a parabolic path. I A surface of parabolic cylindrical curvature has therefore a focal line defined by the trace of the foci of the parabolic cylindrical surface and a vertex line defined by the trace of the vertices of the parabolic cylindrical'surface and an axial plane defined by the focal and vertex lines. The paraboloid l2 and the cylinder [3 are oriented so that the axis of the paraboloid I 2 is in the axial plane of the parabolic cylinder l3.

The type of radiation pattern which is produced by this novel antenna is indicated in Fig. 3 and closely approximates the relation csc 0 where 0 is the radiation angle measured from the radiation axis 9:0. 7

Lower portion 13 of the reflector II is secured to depend from upper portion I2 in anysuitable manner such as by rivets or bolts l3a so that the line of intersection or attachment between upper and lower portions I 2 and [3 may be substantially on the line of intersection of the paraboloidal upper portion l2 and a horizontal plane parallel to the axis of paraboloidal portion l2. Preferably, however, the line of intersection follows along a current line in the E plane, that is, on a curved line of intersection of upper portion l2 and a plane through the focal point of upper portion l2 and angularly related to the focal axis of portion l2. While the outer edge of the lower portion l3 contributes relatively little to the radiation pattern, the shape of the cylindrical portion I3 is preferably such that a projection of reflector II on a plane perpendicular to its axis is substantially a circle.

This preferred arrangement is shown in the reflector surface outline drawing of Fig. 4 which differs from the Fig. 2 arrangement only in the edge shape of paraboloid l2 where cylinder [3 plane polarized with the electric vector normal to the drawing sheet, then for the Fig. i arrangement, the plane edge of the truncated paraboloid I2 lies in an electric vector plane of the radiation from the feed unit II]. I h 7 An important factor is the position of the line of intersection of lower portion I-3 with upper portion I2 relative to the reflector and axis, that is the distance D (Fig. 2 and Fig. 4); "since the gain is in direct proportion to the distance D and as this determines to a large extent the amount of antenna tilt for which the pattern conforms with a csc 0 relation. This tilt,- in connection with the maximum range of the system, determines the altitude for which the antenna gives optimurrr coverage since h'=R sin a, where h'= height, R-=slant range, and a is the angle with'the horizon. It has been determined that the distance D is approximately of the diameter of reflector I 2. Thus for a reflector hav ing a diameter of I8", the optimum distance D may be chosen as 3.5".

Other important factors in the construction of the reflector according to this invention are phasing of the reflected waves of energy, tilt of lowerportion I3 withrespect to upper portion I2 and'the distance of the center of feed or radiatin'g element I0 from the vertex of paraboloid I2.

For best phasing of the reflected energy it is preferable that there be discontinuity between upper and lower portions I2 and I3 at their line of attachment or intersection. Thus, it is desirable that lower portion I3 be placed slightly behind the upper portion I2 to minimize or avoid the development of holes and high lobes on the non-csc o side of the pattern. It has been found that best results are obtained when lower portion I3 is disposed so that its upper edge" is spaced frcmthe rear lower edge portion of up-' per portion I2, as at I4, Fig. 2, by about /40 of the wavelength in the 3 centimeter band of energy frequencies, or .040" for an 18" reflector as herein described. When this spacing is slightly greater, the radiation pattern is less sensitive to irregularities of the reflector and the feed but this advantage is offset by loss of gain. It should be noted that although the lower portion I3 is spaced behind upper portion I2 the com bined reflectin surface or front surface presented toward the radiating element I 0, due to the slight overlapping relation thereof, appears as an unbroken surface;

While the tilt of lower portion I3 relative to upper portion I2 is less critical, it is preferable that lower portion I3 be tilted to assume an angle a (Fig; 2 andFig. 4) about one or two degrees back of a line tangent to the surface of upper portion I 2 at I4. That is, the vertex line forms an acute angle a with theaxis of paraboloid I2 which is preferably greater than the acute angle 8 formed by the tangent line. For the 18" reflector referred to above, lower portion I3 would assume an angle q of approximately 75 with a horizontal plane through the axis of re flector II; A smaller tilt angle (forward from tangency) has the effect of developing holes and high lobes begin to appear on the csc o side of the radiation pattern and for tilts more than 3 or 4 degrees back of tangency the energy power distribution at small angles begins to decrease.

It is preferable that the lower portion I3 be parabolic and so dimensioned that its focal length be proportional to that cf upper portion I2. It has been determined that the best focal length of lower portion I3 is that which is determined when the focal length of lower section I3 is equal to that of upper portion I2 divided by the sine of the angle between the axes of upper portion I2 and lower portion I3. In the case of an 18" reflector, the focal length is If ,the vertex line of the parabolic cylinder lies substantially along a tangent to the paraboloid and if the axial plane of the parabolic cylinder contains the axis of the paraboloid and if t he focal length of the parabolic cylinder is equal to that of the paraboloid divided by sin a can be proved mathematically that the focal line of the parabolic cylinder will pass through or near the focus of the paraboloid.

For obtaining a good compromise between high gain and good radiation pattern for long range it is preferable that the distance of the center of radiation of feed I0 from the vertex I5 of re:- flector I I be shorter than the focal length of the paraboloidal upper portion I2. For the 18" refiector the distance from vertex I5 to the endof the wave guide for a two dipole radiating element is approximately 5.0" for the best compromise.

In airborne antennas of the kind described, it is usual practice to' provide a, pressurizin' cup for maintaining equal pressure around the radiation or feed elements. In certain cases the pressur'izing cup has a detrimental eirectcm the radiation pattern sometimes causing bad holes and increasing the size of unwanted lobes. It has been found that these difficulties may be substantially overcome by providing suitably phased irregularities of the order of one-eighth of the wavelength of the radiant energy on the reflecting surface of the reflector. According to one embbdiment of the invention, such irregularities comprise two fiat sheet-like members I6 and I! of conducting material such as brass and preferably of rectangular shape. Members I6 and H are mounted substantially symmetrically on opposite edge portions of the upper portion I2 in the E-plane in any suitable manner such as by screws I8. The optimum position of members Iliarrd I1 at the extremities of reflector II in the E-plane is found to be when members I6 and I! are spaced out from reflector I I parallel to the axis of the paraboloid about one-sixth of the wavelength in a plane parallel to tangency at the point of attachment with the long dimer}: sibns of the rectangularly shaped members I6 and I! in the H-plane. 7

It will be appreciated that according to the invention as described above the paraboloidal up-' per portion I2 tends to direct radiant energy ahead of reflector I I in a pencil type beam while the lower portion I3 tends to deflect the radiation downwardly. The combination of these effects is such as to produce the csc 0 radiation pattern as described, and substantially as illiis= trated in Fig; 3. With the structure as de-' scribed the antenna adapted to afford an energy distribution pattern, suitable for long range navigation having a relatively high gain such as an absolute gain of the order of 700. It should be noted that certain modifications may be made in theshape of the reflector ll, if desired, to alter the cosecant-squared pattern.

From the above description it will be evident that, in accordance with the present invention,-

there is provided an antenna structure for transmitting and receiving radio frequency energy waves and preferably waves which are polarized in a chosen plane. The antenna structure is adapted to provide a fan-shaped directional pattern which is characterized in a plane normal to the plane of "polarization by an asymmetrical major lobe which corresponds to a cosecant squared energy distribution. The structure comprises a feed element for transmitting plane polarized waves, which element is effectively located at the focus of a reflector. The reflector comprises a first portion having paraboloidal curvature and truncated to provide an edge which lies in a plane cutting the paraboloid at a predetermined distance from the paraboloidal axis. Preferably the plane also cuts the axis of the paraboloid at the focus and is oriented to include the electric vector at the focus. A second portion in the form of a parabolic cylinder is attached to the first portion at the plane edge by overlapping in back thereof and the relative arrangement and dimensions of the two portions are such that the axis of the first lies in the axial plane of the second and the focus of the first lies on or near the focal line of the second.

Preferably in accordance with the invention the overlap of the first and second portions of the reflector is chosen to provide at the plane edge on the reflector. surface a discontinuity or step in the electric vector plane of an amount corresponding to /40 of a wavelength of the radio frequency energy waves. Furthermore, the predetermined distance between the paraboloidal axis and the lowest point at which the paraboloid is truncated is chosen to be 20% of the outer diameter of the first portion and the vertex line of the second Portion is preferably inclined at an acute angle to the paraboloidal axis which is slightly greater than the acute angle made by a line lying in the axial plane of the second portion and tangent to the paraboloid at the abovementioned plane edge.

Also in accordance with the invention there is employed a pair of auxiliary reflector surfaces which are symmetrically disposed relative to the axial plane of the parabolic cylinder and located in front of the paraboloidal portion and near the ends of a diameter thereof and spaced from the surface thereof by approximately /6 of a wavelength.

While there has been described certain embodiments of the present invention it will be manifest to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. A reflector for use with a radiating element for transmitting and receiving radio frequency energy and adapted to provide a fan shaped directional pattern characterized by an asymmetrical major lobe, said reflector comprising a first dish shaped portion having paraboloidal curvature with a vertex and a focus and an axis therebetween and truncated to provide an edge which lies in a plane cutting said dish at a predetermined distance from the paraboloidal axis, and

a second spade shaped portion having parabolic cylindrical curvature with an axial plane defined by the focal line and the, vertex line thereof and attached to the first portion at said edge; the relative arrangement of said portions being such that the axis of the first lies in said axial plane of the second.

2. A reflector for use with a radiating element for transmitting and receiving radio frequency energy and adapted to provide a fan shaped directional pattern characterized by an asymmetrical major lobe, said reflector comprising a first. dish shaped portion having paraboloidal curvature with a vertex and a focus and an axis therebetween and truncated to provide an edge which lies in a plane cutting said dish at a predetermined distance from the paraboloidal axis and a second spade shaped portion having parabolic cylindrical curvature with an axial plane defined by the focal line and the vertex line thereof and attached to the first portion at said edge by overlapping in back thereof; the relative arrangement of said portions being such that the axis of the first lies in said axial plane of the second.

3. A reflector in accordance with claim 2 in which said overlap is chosen to provide at said edge a discontinuity between the surfaces of said first and second reflector portions of the order of /40 of the wave length of said radio frequency energy, said predetermined distance being chosen to be approximately 20% of the outer diameter of said dish portion and the vertex line of said second portion being inclined at an acute angle to the axis of said first portion which is slightly greater than the acute angle made with the axis ofsaid first portion by a line lying in said axial plane of said second portion and tangent to said first portion at said edge.

4. A reflector in accordance with claim 3 in which a pair of auxiliary reflector surfaces are symmetrically disposed relative to said axial plane of said second portion, in front of said paraboloidal surface, spaced therefrom by approximately /6 of said wave length and located r near the ends of a diameter of said paraboloidal surface.

5. A reflector for use with a radiating element for transmitting and receiving radio frequency energy and adapted to provide a fan shaped directional pattern characterized by an asymmetrical major lobe corresponding to a cosecant squared energy distribution, said reflector comprising a first dish shaped portion having paraboloidal curvature with a vertex and a focus and an axis therebetween and truncated to provide an edge which lies in a plane cutting said dish at a predetermined distance from the paraboloidal axis, said plane being parallel to said axis and a second spade shaped portion having parabolic cylindrical curvature with an axial plane defined by the focal line and the vertex line thereof and attached to the first portion at said edge by overlapping in back thereof; the relative arrangement of said portions being such that the axis of the first lies in said axial plane of the second and the focus of the first lies in the focal line of the second.

6. A reflector for use with a radiating element for transmitting and receiving radio frequency energy and adapted to provide a fan shaped directional pattern characterized by an asymmetrical major lobe corresponding to a cosecant square energy distribution, said reflector comprising a first dish shaped portion having paraboloidal curvature with a vertex and a focus and an axis thereb'eti'veen and truncated to provide an edge which lies in a plane cutting said dish at a predetermined distance from the paraboloidal axis, said plane also cutting said axis at the focus and a second spade shaped portion having parabolic cylindrical curvature with an axial plane defined by the focal line and the vertex line thereof and attached to the first portion at said edge by overlapping in back thereof; the relative arrangement of said portions being such that the axis of the first lies in said axial plane of the second and the focus of the first lies in the focal line of the second.

'7. A reflector in accordance with claim 6 in which said overlap is chosen to provide at said edge a discontinuity or step in the electric vector plane between the surfaces of said first and second reflector portions of the order of /40 of wave length of said radio frequency energy waves, said predetermined distance being chosen to be 20% of the outer diameter of said dish portion, and the vertex line of said second portion being inclined at an acute angle to the axis of said first portion which is slightly greater than the acute angle made with the axis of said first portion by a line lying in said axial plane of said second portion and tangent to said first portion at said edge, and auxiliary reflector means critically located in front of said reflector surface for compensating for irregularities in said directional pattern.

8. A reflector for use with a radiating element for transmitting and receiving radio frequency energy waves polarized in a chosen plane and adapted to provide a fan shaped directional pattern characterized in a plane normal to said polarization plane by an asymmetrical major lobe corresponding to a cosecant square energy distribution, said reflector comprising a first dish shaped portion having paraboloidal curvature with a vertex and a focus and an axis therebetween and truncated to provide an edge which lies in a plane cutting said dish at a distance from the paraboloidal axis, said plane also cutting said axis at the focus and being oriented to out said dish in the electric vector plane and a second spade shaped portion having parabolic cylindrical curvature with an axial plane defined by the focal line and the vertex line thereof and attached to the first portion at said edge by over lapping in back thereof; the relative arrange ment of said portions being such that the axis of the first lies in said axial plane of the second and the focus of the first lies in the focal line of the second.

9. A reflector in accordance with claim 8 in which said overlap is chosen to provide at said edge a discontinuity or step in the electric vector plane between the surfaces of said first and sec ond reflector portions of the order of /40 of wave length of said radio frequency energy waves, the distance between the axis of said dish-shaped portion and the point in said axial plane at which said dish-shaped portion is truncated being chosen to be 20% of the outer diameter of said dish portion, and the vertex line of said second portion being inclined at an acute angle to said axis of said first portion which is slightly greater than the acute angle made with the axis of said first portion by a line lying in said axial plane of said second portion and tangent to said first portion at said edge.

10. A reflector in accordance with claim 9 in which a pair of auxiliary reflector surfaces are symmetrically disposed relative to said axial plane of said second portion in front of said paraboloidal portion near the ends of a diameter thereof and spaced from the surface thereof by approximately /6 of said wave length.

11. A reflector according to claim 8 in which at least one auxiliary reflecting surface is disposed adjacent one of said first and second portions on the operative side thereof symmetrically with respect to the axial plane of said second portion to provide an irregularity in said one portion.

JOHN H. GARDNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,061,508 Dallenbach Nov. 17, 1936 2,095,083 Renatus Oct. 5, 1937 2.160.853 Gerhard et al. June 5, 1939 

